Voting regulation is not a power expressed to Congress (except as related to ensuring no bias in race, age, and gender), so his unilateral action has no basis in law. But, when has a basis in law ever applied to him? Lawful grounds such as ‘consent’ has never applied to him before. Why would it now?
To append to my other comment:
He has even less authority over midterms than he does over the presidential election. Which is to say none, but even indirectly, far less.
Even if there was a question of negation on state votes for congresspeople, it would not be, in any way shape or form, the executive’s prerogative. It would be each house of Congress itself. It’s like parliament vs the monarch.